As is well known, Stirling cycle cryogenic refrigerators, or cryocoolers, use a motor driven compressor to impart a cyclical volume variation in a working volume filled with pressurized refrigeration gas. The pressurized refrigeration gas is fed from the working volume to one end of a sealed cylinder called a cold head. An annular heat exchanger or regenerator is positioned inside the cold head. The regenerator has openings in either end to allow the refrigeration gas to enter and exit.
The compressor and expander reciprocate in a fixed relationship creating the volume variations in the working space and forcing the refrigeration gas to flow through the regenerator in alternating directions. One end of the regenerator is above ambient temperature during operation while the other end is at a cryogenic temperature. Gas enters the expander at cryogenic temperature and as the gas expands it absorbs heat, ideally, at constant temperature. The device to be cooled is mounted adjacent the expansion space, on the cold end of the cold head.
Because the cold head is sealed, the volume of the expansion space also varies as the expander reciprocates. The efficiency of a Stirling cryocooler is optimized by properly timing the movement of the expander. Specifically, its movement should be such that the variations in the volume of the expansion space lead the variations in the volume of the compression space by approximately 90.degree.. This insures that The working volume's pressure and temperature are at a peak before the refrigeration gas enters the regenerator from the working volume.
The two most common configurations of Stirling cryocoolers are referred to as "split" and "integral". The split Stirling type has a compressor which is mechanically isolated from the expander. Cyclically varying pressurized gas is fed between the compressor and expander through a gas transfer line. In most split Stirling cryocoolers proper timing of expander movement is achieved by using precision friction seals.
In an integral Stirling cryocooler, the compressor, heat exchangers, regenerator and cold head are assembled in a common housing. The typical arrangement uses an electric motor to drive the moving parts. A crankshaft, disposed in a crankcase, is used to properly time compressor and expander movement, much as an internal combustion engine uses a crankshaft to provide proper timing of the movement of its parts. As such, the typical integral cryocooler requires several bearings to support the crankshaft. If connecting rods are used to couple the compressor and expander to the crankshaft, additional bearings are required. One problem with this arrangement is that these bearings require a lubricant. Unfortunately, lubricants are subject to freezing at cryogenic temperatures and consequently must be prevented from freezing and plugging the regenerator. Many different sealing arrangements have been used. Some Stirling systems use contact seals of the wearing type along with hydro formed bellows to prevent lubricant from reaching the regenerator. However, these arrangements produce wear particles which result in limited operating life.
One way to prevent oil containing refrigerant gas in the crankcase from reaching the oil-free refrigerant gas in the regenerator is to use a bellows seal. Bellows seals have been found to be particularly suited for this application. The bellows configurations have been stacked or axially spaced and have excessive height/length requirements.